1. Field of the Invention
The present invention relates to switching control circuit, and more particularly, to a switching control circuit of a magnetic device.
2. Description of Related Art
Switching control circuits have been widely used in power conversion. FIG. 1 shows a power converter including a controller 50 for generating a switching signal SW to regulate the output of the power converter in response to a feedback signal VFB. The switching signal SW drives a power transistor 20 for switching a transformer 10. The transformer 10 is connected to an input voltage VIN of the power converter. The energy of the transformer 10 is transferred to the output voltage VO of the power converter through a rectifier 40 and a capacitor 45. A resistor RS is connected in series with the power transistor 20 to generate a current signal VI in response to a switching current IP of the transformer 10. The current signal VI is coupled to the controller 50 to control the power converter. The power converter may be operated in a discontinuous current mode (DCM) when the magnetic device such as the transformer 10 is fully discharged before the start of the switching cycle. If the switching signal SW is enabled before the transformer 10 is fully discharged, the power converter may be operated in a continuous current mode (CCM). A continuous current may be retained in the transformer when the power converter operated in the CCM. FIG. 2 shows a CCM waveform of the switching current IP, in which the continuous current IA represents the energy stored in the transformer 10. A current IB is the energy that is further charged into the transformer 10 during the on time TON of this switching cycle. The continuous current IA stands for a major energy transfer of the transformer 10. The object of the present invention is to develop a method and apparatus to measure the continuous current IA of the switching current IP.